Download Polymer laser fabricated by a simple micromolding process

APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 23
9 JUNE 2003
Polymer laser fabricated by a simple micromolding process
Justin R. Lawrence, Graham A. Turnbull, and Ifor D. W. Samuela)
Ultrafast Photonics Collaboration and Organic Semiconductor Centre, School of Physics and Astronomy,
University of St. Andrews, North Haugh, Fife, KY16 9SS United Kingdom
共Received 3 February 2003; accepted 3 April 2003兲
We report polymer distributed feedback lasers fabricated using solvent-assisted microcontact
molding. The poly关2-methoxy-5-共3,7-dimethyloctyloxy兲 paraphenylenevinylene兴 film is patterned
by placing it in conformal contact with an elastomeric mould inked with a suitable solvent. When
the resulting microstructured film is pumped with the 532 nm pulsed output of a microchip laser, we
observe lasing above a threshold pump energy of 225 nJ. Above threshold the emission narrows to
a linewidth of less than 0.6 nm at a wavelength of 638 nm. This micromolding technique may find
application to a wide range of wavelength-scale microstructured organic photonic devices. © 2003
American Institute of Physics. 关DOI: 10.1063/1.1579858兴
Wavelength-scale microstructures can be used to give
remarkable control over the emission and propagation of
light in semiconductors. This has led to many recent advances in the field of photonic crystals,1,2 including work
towards the development of integrated photonic circuits and
devices. The fabrication of such microstructures, however, is
demanding, and often requires elaborate techniques such as
electron beam lithography followed by chemical or ion etching. Such complicated processing limits the viability of mass
production of wavelength-scale microstructured photonic devices.
One family of semiconductors—organic semiconductors—offers the possibility of much simpler fabrication and processing of photonic devices. This is proving to
be a very attractive medium for a range of photonic devices
including light-emitting diodes, lasers, and photovoltaics.
Within this family, conjugated polymers are particularly appealing because of their high photoluminescence 共PL兲 quantum yield3–5 and ability to be processed from solution.
One application of microstructured semiconducting
polymers has been the distributed feedback 共DFB兲 laser.
Most fabrication to date has involved spin-coating the polymer onto microstructured silica substrates to form DFB lasers of various geometries including single gratings, crossed
gratings, and circular gratings.6 –9 These substrates are usually patterned by standard lithographic techniques, which remove the key advantages of simple fabrication. To address
this issue, microstructured polymer substrates have previously been replicated for DFB lasers using an UV embossing
process.10,11 However, this approach limits the choice of substrate material, which may limit the eventual inclusion of
electrical contacts into the device.
An alternative approach is to directly pattern the active
polymer. We have previously demonstrated a microstructured
conjugated polymer film formed by hot embossing
lithography.12 This technique can reliably replicate feature
sizes below 100 nm, but requires the use of relatively high
temperatures that may effect the PL quantum yield and other
optical properties of the polymer. A potentially less harmful
fabrication method that allows microstructure to be directly
imprinted on the active polymer layer would therefore be
very attractive.
In this letter we show how the technique of solvent assisted microcontact molding 共SAMIM兲13 provides a simple
way of directly patterning semiconducting polymer films
with useful wavelength scale structure. This approach does
not involve the use of heat or pressure and so is a particularly
fast and simple way of making photonic structures. The
structures formed are characterized using atomic force microscopy and the effect of solvent choice and imprinting time
are examined. To demonstrate the potential of applying this
process to conjugated polymers, we have used it to fabricate
polymer DFB lasers and studied their operating characteristics. In doing so, we have demonstrated a simple, single step
process in which a semiconducting polymer film can be
turned into a DFB laser in less than 2 min.
The conjugated polymer used was poly关2-methoxy-5共3,7-dimethyloctyloxy兲 paraphenylenevinylene兴 (OC1 C10 –
PPV) and its structure is shown in Fig. 1. Thin films of
OC1 C10 – PPV were formed by spin coating from chlorobenzene solution onto glass substrates, resulting in films of
about 100 nm in thickness. The air-polymer-glass structures
formed asymmetric slab waveguides that support only the
fundamental TE mode within the polymer’s emission bandwidth. Thicker polymer films were formed by drop casting
from chlorobenzene solution.
a兲
FIG. 1. Chemical structure of the polymer OC1 C10 – PPV and an atomic
force microscope image of a polymer film imprinted with a 6 ␮m grating.
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0003-6951/2003/82(23)/4023/3/$20.00
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© 2003 American Institute of Physics
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Appl. Phys. Lett., Vol. 82, No. 23, 9 June 2003
FIG. 2. Depth of imprinted structure as a function of imprinting time.
These films were patterned using an elastomeric mold13
to transfer the desired pattern from an etched silica master to
the conjugated polymer film. A liquid prepolymer Sylgard
184 共Dow Corning兲 was poured over the master and allowed
to cure for 48 h. The cured elastomer was later separated
from the master. The stamp was subsequently inked by applying a small amount of a solvent using a Q-tip. The spincoated polymer film was placed in contact with the inked
stamp. As the stamp material is elastomeric, conformal contact was easily made with the polymer film. The solvent on
the stamp dissolved the polymer, allowing it to flow around
the microstructure and conform to its shape. Once the solvent had evaporated the stamp and patterned polymer film
were separated. Importantly with this micromolding technique no heat or pressure need be applied to the polymer,
which may affect its optical properties.
The choice of solvent is critical to the micromolding
process. It should dissolve the polymer to be patterned without swelling the elastomeric mold. Nonpolar solvents cause
the least swelling in poly共dimethylsiloxane兲 共PDMS兲.14 Tetrahydrofuran, acetone, and toluene were tried but were either
poor solvents for the polymer or caused swelling of the
mold, which caused the polymer film to stick. Chlorobenzene was found to give the best results. An atomic force
microscope image of a 1-␮m-thick polymer film imprinted
with a linear grating is shown in Fig. 1. The period of the
master 共6 ␮m兲 has been reproduced exactly while the depth
of the imprinted structure is 300 nm. The depth that can be
achieved depends on the imprinting time. Figure 2 shows the
depth of a 400 nm period grating imprinted into a 120-nmthick OC1 C10 – PPV film as a function of imprint time. The
maximum depth 共20 nm兲 is reached after about 100 s.
To demonstrate the usefulness of this technique we fabricated distributed feedback polymer lasers. A 120-nm-thick
(OC1 C10 – PPV) film was imprinted for 100 s using chlorobenzene. The structure was a 400 nm crossed grating
which provides both distributed laser feedback and a surface
emitted output coupling via second and first order Bragg
scattering, respectively. An atomic force microscopy image
of the film is shown in Fig. 3. There have been a number of
reports of submicron scale linear structures formed by
SAMIM but the features tend to collapse and merge during
imprinting.15 In our work the imprinted structure is a crossed
Lawrence, Turnbull, and Samuel
FIG. 3. Atomic force microscope image of a polymer film patterned with a
400 nm period DFB structure.
grating which may provide support that prevents these defects.
The patterned polymer films were mounted in a vacuum
chamber and held under a pressure of 10⫺4 mbar to isolate
them from oxygen and water. The films were optically
pumped using a frequency-doubled passively Q-switched
Nd:YVO4 microchip laser 共Alphalas GmbH兲 which produced 1 ns pulses at 532 nm with a repetition rate of 5 kHz.
The output of the laser beam was attenuated using neutral
density filters then focused using a 63 mm focal length
spherical lens to a spot of half width 100 ␮m 共horizontal兲 by
80 ␮m 共vertical兲. The pump beam was aligned at 20° to the
film normal. The spectral output and the output energy from
the films were measured in a direction close to normal incidence using a fiber-coupled charge coupled device camera.
The emission spectrum of an optically pumped polymer
film is shown in Fig. 4 for low 共solid line兲 and high 共dotted
line兲 pump intensities. Below threshold 共75% of the threshold energy兲 the emission is broad and shows Bragg scattered
features.12 Above threshold 共200%兲 the emission narrows to
a linewidth of 0.6 nm 共limited by instrument resolution兲 at a
wavelength of 638 nm. Figure 5 shows the output energy of
a laser as a function of pump energy. The threshold for laser
oscillation was 225 nJ and the energy of the surface emitted
output was 0.1 nJ for a pump energy of 450 nJ. We expect
that in future work it should be possible to reduce the thresh-
FIG. 4. Emission spectrum of a micromolded polymer film pumped above
共dotted line兲 and below 共solid line兲 threshold.
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Appl. Phys. Lett., Vol. 82, No. 23, 9 June 2003
Lawrence, Turnbull, and Samuel
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The authors are grateful to the EPSRC and SHEFC for
financial support, Covion for the supply of OC1 C10 – PPV
and P. Andrew, W. L. Barnes, R. Fisackerly, T. F. Krauss, and
Rutherford Appleton Labs for the fabrication of etched silica
masters. I.D.W.S. is a Royal Society University Research
Fellow; G.A.T. is an EPSRC Advanced Research Fellow.
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1
2
FIG. 5. Output energy from a polymer laser as a function of pump energy.
old by increasing the corrugation depth from its present
value of 15 nm. For example in similar SiO2 structures with
a grating depth of 100 nm, a lasing threshold of 4 nJ was
observed.16 Ongoing experiments aiming at increasing this
depth should therefore significantly improve the performance
of the laser.
In summary, we have shown that solvent-assisted microcontact molding is a simple and effective method for fabricating wavelength-scale microstructures. Using this technique, a conjugated polymer film was patterned with a 400
nm crossed grating structure which provides both optical
feedback and weak surface output coupling. Distributed
feedback lasers have been demonstrated which have a threshold of 225 nJ. This simple processing technique is compatible with a wide range of substrate materials and could lead
to a range of microstructured polymer devices.
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